1. Field of the Invention
The present invention relates to a decoding apparatus and method, and more particularly, is applicable to a decoding apparatus and method for decoding video data compressive-encoded by the moving picture experts group phase 2 (MPEG2) system.
2. Description of the Related Art
Various compressive-encoding systems have been proposed as a method of reducing the amount of video data. The MPEG2 system has been especially used widely. The MPEG2 standard is practically the international standard which intends to provide the general-purpose encoding of video data and audio data. The MPEG2 standard has been used in many fields such as broadcasting, communication, and storage medium.
In the MPEG2 standard, five profiles (functionality) and four levels (size of image, etc.) are provided as typical specifications to keep the mutual compatibility of bit stream. The most useful profile and level are set as main profile and main level, respectively.
In the MPEG2 standard, there is generally no provision with respect to the concrete contents for decoding the compressive-encoded video data and with respect to the memory size needed at a decoding side. These practically depend on the setting of the profile and level by which the image data was encoded. More specifically, at a decoding apparatus side, for the size of a buffer memory for storing input signals temporally or the size of a frame memory used for storing reference image data and rearranging decoded images, suitable value for decoding without the occurrence of error is determined by the profile and level in encoding.
However, in the MPEG2 standard, as the level of the video data in encoding becomes higher, the number of pixels in the horizontal direction and the vertical direction operated in decoding increases. For this reason, in the decoding apparatus for decoding the video data encoded with high level of the MPEG2, there is a problem that the required size of frame memory for storing reference image data and rearranging images becomes larger in addition to the necessity of high-speed decoding circuit, so that the manufacturing cost becomes expensive.
As one method of solving the above problem, a decoding method has been proposed conventionally, the decoding method in which a decoding processing such as the variable-length decoding and the inverse discrete cosine transform (DCT) is changed in its processing contents so as to decode the video data, which has been compressive-encoded with the main-profile-at-high-level of the MPEG2 Standard, with the memory size and operation speed of the main-profile-at-main-level.
However, this decoding method has a problem that another circuit which is different from the original decoding circuit is newly needed since the method itself differs from the pure decoding processing of the MPEG2, so that the design cost and manufacturing cost are required for this.
Further, this decoding method has a problem of low functionality, such like that the high-leveled encoded data that the higher operation speed and larger memory size are further required can not be decoded, reproduced and output with the required quality while keeping the high level.
Accordingly, in the decoding apparatus, if the encoded data from high level to low level can be decoded surely without change of the decoding contents while reducing the size of the frame memory for storing reference image data and for rearranging images as much as possible, the above-described problems are all solved and the decoding apparatus which has high functionality, simple construction, and low manufacturing cost may be constructed.
In view of the foregoing, an object of this invention is to provide a decoding apparatus and method which can improve the functionality and simplify the construction.
The foregoing object and other objects of the invention have been achieved by the provision of a decoding apparatus which comprises a compressing means for compressing the image data decoded by a decoding means and supplying it to storing means, and a decompression (referred to interchangeably as xe2x80x9cextendingxe2x80x9d) means for decompressing the compressed image data stored in the storing means and supplying the decompressed image data to the decoding means.
As a result, the decoding apparatus can avoid the complication of the whole construction and can perform the decoding processing from high level to low level while reducing the size of the storing means efficiently, since the decoded image data is compressed and then stored in the storing means.
Further, according to this invention, the decoding method comprises a first step of compressing the image data obtained by decoding the encoded data and storing it in the storing means, and a second step of reading and extending the compressed image data stored in the storing means and decoding the encoded data based on the extended image data.
As a result, the decoding method can deal with the decoding processing from high level to low level while reducing the size of the storing means efficiently, since the decoded image data is compressed and then stored in the storing means.